Diaz’s mother was rescued hours after the abduction in the northern town of Barrancas last weekend but his father Luis Manuel Diaz is still missing.
The Colombian government on Thursday accused the ELN guerrilla group of being behind the kidnapping, which took place in their home town.
The ELN and the government of leftist president Gustavo Petro are in the midst of peace negotiations and a six-month ceasefire, which entered into force in August.
The ELN, or National Liberation Army, has not claimed responsibility for the kidnapping.
Klopp said 26-year-old winger Diaz had trained this week ahead of the match at Luton on Sunday.
“The session he had with us, you can see when he is with the boys he is fine, he is OK, but you could see he didn’t sleep a lot.
“We have to see how he is and we go from there. The news from Colombia gives us a little bit of hope. We are waiting for the really good news but that’s it pretty much.
“I cannot say what we will do because we just wait where we can pick up the boy from and go from there.
“But it’s all about him – if he makes himself available or not, and I will not force anything.”